The use of metallocene compositions in olefin polymerization is well known. Metallocenes containing substituted, bridged indenyl derivatives are noted for their ability to produce isotactic propylene polymers having high isotacticity and narrow molecular weight distribution. Considerable effort has been made toward obtaining metallocene produced propylene polymers having ever-higher molecular weight and melting point and, thus, ever better strength (impact) properties, while maintaining suitable catalyst activity.
Toward this end researchers have found that there is a direct relationship between the way in which a metallocene is substituted and the molecular structure of the resulting polymer. For the substituted, bridged indenyl type metallocenes, it is now well established that the type and arrangement of substituents on the indenyl groups, as well as the type of bridge connecting the indenyl groups, determines such polymer attributes as molecular weight and melting point.
For example, U.S. Pat. Nos. 5,840,644 and 5,770,753, incorporated herein by reference in their entireties, describe certain metallocenes containing aryl-substituted indenyl derivatives as ligands, which are said to provide propylene polymers having high isotacticity, narrow molecular weight distribution and very high molecular weight.
Likewise, U.S. Pat. No. 5,936,053, incorporated herein by reference in its entirety, describes certain metallocene compounds said to be useful for producing high molecular weight propylene polymers. These metallocenes have a specific hydrocarbon substituent at the 2 position and an unsubstituted aryl substituent at the 4 position on each indenyl group of the metallocene compound.
WO 98/40419 and WO 99/42497 both describe certain supported catalyst systems for producing propylene polymers having high melting point. Metallocene compositions and their activators are often combined with a support material in order to obtain a catalyst system that is less likely to cause reactor fouling. However, it is known that supported metallocene catalyst systems tend to result in a polymer having lower melting point than would otherwise be obtained if the metallocene were not supported.
Much of the current research in this area has been directed toward using metallocene catalyst systems under commercially relevant process conditions, to obtain propylene polymers having melting points higher than known metallocene catalyst systems and close to, or as high as, propylene polymers obtained using conventional, Ziegler-Natta catalyst systems. The present inventors have discovered metallocene compounds that not only have this capability, but retain it upon supportation.
Additionally, it would be desirable to have available metallocenes which not only afford propylene homopolymers having high melting points of (i.e., high stereotacticity), but also elastomeric copolymers having the high molecular weights required for the production of, e.g., impact copolymers, thereby making possible the production of satisfactory in situ blends of, e.g., propylene homopolymer and ethylene-propylene rubbers (EPR's) with a single catalyst composition in a single reactor or in a series of two or more reactors. The present inventors have found metallocene compounds which in combination with a cocatalyst (activator) make both propylene homopolymers having high melting points and elastomeric copolymers that are suitable for the production of impact copolymers in combination with these propylene homopolymers.